Hydrogen is used in the manufacture of many products including edible fats and oils, metals, semiconductors and microelectronics. Hydrogen is also an important fuel source for various energy conversion devices. For example, many types of fuel cells use purified hydrogen and an oxidant to produce electrical energy.
Various processes and equipment are used to produce hydrogen that is consumed by fuel cells. One such piece of equipment is a steam reformer, which reacts water and a hydrocarbonaceous material, such as an alcohol feed in the presence of a steam reforming catalyst to produce a reformate comprised predominantly of hydrogen.
Although catalysts in powder form can be used in chemical process units, catalyst particles are typically formed into shapes such as spheres, pellets and rods. While these shapes are easier to handle, the result in usually a decrease in catalyst activity and/or selectivity.
With diminishing liquid fossil fuel reserves, and the world dependent on such fuels for energy with existing fuel consumption equipment design, infrastructure, and logistics designed for such liquid fuels, it has become increasingly desirable to convert vast reserves of natural gas to liquid fuels. Natural gas is comprised mainly of methane, but it is under-utilized due to transportation costs and economic reasons. For example, approximately 50% of the known natural gas deposits in the world (worth trillions of dollars) are in abandoned fields. These fields have significant natural gas deposits, but are located in remote areas, and the amount of reserves does not justify the costs of constructing a transmission pipeline.
Another source of underutilized natural gas is at oil wells, where natural gas is a component of the recovered hydrocarbons. In subterranean oil reserves, the top layer is gas, and though the oil well is constructed to tap into the liquid oil, much of the gas comes to the surface as what is termed associated gas. Typically, the associated gas is flared, except in instances where the oilfield is close to a major gas pipeline.
Gas to liquids (GTL) using the well-known Fischer-Tropsch (FT) reaction has received a great deal of attention in the last few decades. The Fischer-Tropsch process involves a series of chemical reactions that result in the production of a variety of hydrocarbon molecules. The FT process is also one of the most high profile ways to produce synthetic liquid fuel. It converts a mixture of carbon monoxide and hydrogen (syngas) into liquid hydrocarbons of various carbon lengths, such as waxes, paraffins, synthetic diesel and jet fuel.
One of the major issues with the FT process is the rapid increase in temperature after the reaction is initiated. Such a condition needs to be controlled by cooling the reactor at the same rate. This problem has been addressed by many unique reactor designs, but typically requires complicated auxiliary equipment, some of which are redundant for safety reasons.
Another shortcoming of conventional FT processes is the fact that the reaction, by nature, produces about 12 to 34 moles of water for every mole of long chain hydrocarbon, depending on the chain length. This water of reaction is absorbed by the catalyst substrate, which may be at a temperature lower than the boiling temperature of water under the typically high operating pressure of the reactor. This water contributes to deactivation of the catalyst, resulting in frequent catalyst changes/reactivation procedures. The largest component of any GTL plant is the turnover frequency of the catalyst bed. Catalysis experts attribute this drawback to many factors, one of which is the long contact time between reactant gases and catalyst. Thus, it is desirable to use a catalyst system having the ability of better control of local temperatures.
The typical capital cost of a GTL plant, coupled with high operating cost, makes smaller mobile plants uneconomical. Thus, conventional GTL technology cannot be applied to the vast majority of sources of natural gas mentioned above. The plants can usually only be built where an abundant supply of natural gas is guaranteed for a large plant and for a long period of time, such that the billions of dollars that are required to build the plant, can be justified. For example, a recent plant in Qatar, where natural gas is the major hydrocarbon that can be recovered, a GTL plant was built at a cost of 18 to 21 Billion dollars.
While various catalytic reforming processes exist for producing hydrogen from hydrocarbonaceous feeds, such as alcohol feeds, and various Fischer-Tropsch processes exist for producing liquid synthetic fuels from syngas, there remains a need in the art for improvements in process technology, particularly with respect to catalyst utilization, decrease in catalyst turnover rate, and reaction selectivity.